Oscilloscopes and other types of measurement instruments have controls for adjusting instrument scale parameters so that measurements can be viewed on display screens of the instruments. Typically, a measurement instrument has high measurement sensitivity and high dynamic range. For example, an oscilloscope may have an amplitude measurement sensitivity of one millivolt (mV) and a dynamic range in excess of ten volts. Accordingly, an instrument's amplitude control must provide fine adjustment of the amplitude scale while also providing adjustment spanning a wide amplitude range. In addition, a measurement instrument typically has high measurement bandwidth. A high-bandwidth oscilloscope may provide picosecond time resolution while spanning a time range in excess of one second. Thus, an instrument's time control must provide fine control of the time scale while also providing adjustment spanning a wide time range.
One approach for achieving scale control in a measurement instrument uses two concentric knobs. A first, inner knob provides fine scale control within a limited range that is defined by detents selected using a second, outer knob. Typically, the detents are in the 1-2-5 sequence. In an oscilloscope, the first three detents selectable using the outer knob may be 1mV, 2mV and 5mV, the second three selectable detents may be 10mV, 20mV and 50mV, and so on. Were the 2mV detent chosen by adjusting the outer knob, for example, the inner knob would provide fine scale control within the 1mV to 2mV amplitude range. Alternatively, were a 5 volt detent chosen by adjusting the outer knob, the inner knob would provide fine scale control within the 2 volt to 5 volt amplitude range. While the dual concentric knobs provide fine scale control and also span a wide adjustment range, they are inconvenient to use since a user of the measurement instrument must alternately rotate each of the two knobs to adjust the instrument's scale parameters.
A single knob approach to adjusting scale parameters of a measurement instrument is provided by a rotary pulse generator, or RPG, included on the instrument's control panel. The RPG generates a predefined number of pulses, or ticks, for each rotation of the knob about a shaft. In order to achieve fine scale control, each tick is mapped to a small scale step size. For example, in an oscilloscope, sufficiently fine scale control is achieved with an RPG that generates thirty ticks per 360-degree rotation of the knob, where each tick is mapped to a scale step size of one mV. While fine scale control is achieved, scale control over a wide adjustment range using a mapping having this step size is inconvenient. A user of the instrument in this example would have to provide in excess of thirty rotations of the knob to achieve scale control over a one volt range. The single knob approach also lacks selectable detents, which if present, would enable precise scale control in large steps within the scale adjustment range, as designated by the sequence of detents.
There is a need for a method of controlling scale parameters in a measurement instrument that provides fine scale control and scale control spanning a wide adjustment range, that is convenient to a user of the instrument.